The present invention generally relates to facsimile apparatus and more particularly to a facsimile apparatus adapted to communicate via more than one transmission medium each having different transmission quality.
In the commonly used facsimile communication system designed according to the CCITT G3 standard, a facsimile apparatus in the source terminal transmits a PIX signal representing a coded image data for one page document, and when this is completed, sends an EOP signal indicative of end of transmission of that page. In the destination terminal, a facsimile apparatus receives the PIX signal sequentially, and decodes the transmitted image from the PIX signal thus received In this process of decoding, occurrence of transmission error is monitored in each line.
In the conventional facsimile communication procedure, the facsimile apparatus at the destination terminal returns a signal commonly designated as an RTN signal to the source terminal when the detected transmission error has exceeded a predetermined threshold. The discrimination is made usually when the number of transmitted lines that involved the transmission error has reached a predetermined number such as ten. In response to this RTN signal, the facsimile apparatus in the source terminal terminates the transmission by sending a DCN signal indicative of disconnection of facsimile transmission. At the same time, the facsimile apparatus in the source terminal displays a message such as "RE-SEND THE DOCUMENT" and requests the re-transmission of document to the operator at the source terminal.
In recent years, facsimile communication is made not only between facsimile apparatuses at offices via wired transmission line in which a high transmission quality is guaranteed but also between a facsimile apparatus at a fixed station such as in an office and a facsimile apparatus at a mobile station such as on an automobile, or between two mobile stations via a wireless or radio transmission line. The term "wired transmission line" used herein means a transmission line such as public telephone network wherein a high transmission quality is guaranteed and may involve radio transmission path wherein a high transmission quality is guaranteed such as microwave transmission path or satellite transmission path therein. On the other hand, the phrase "wireless transmission line" means a transmission line such as automobile telephone network which provides a communication service with a lesser transmission quality. Further, there is recently developed a portable facsimile apparatus which can be connected to the public telephone network via an acoustic coupler. When using this portable facsimile apparatus, the acoustic coupler connected to the facsimile apparatus is mounted on a handset of telephone apparatus and the facsimile transmission is made in a form of acoustic signal exchanged between the acoustic coupler and the telephone handset.
In such a newly developed facsimile communication system using the transmission medium such as wireless transmission line or acoustic coupler, the system is naturally vulnerable to noises and thus, there occurs rather easily a case in which transmission error amounting several tens of lines appears in one page document. When this occurs, the conventional facsimile apparatus interrupts the facsimile transmission and requests the operator to re-send the document. Thus, there arises a problem that the operator has to carry out a number of trials in order to send one document when the facsimile transmission is made via wireless transmission line or via acoustic coupler.
This problem may be solved if the error detection at the destination side facsimile apparatus is totally omitted and the destination receives the facsimile transmission whatever the quality of the transmission may be. However, such a measure is unacceptable as there is a substantial chance that an entirely unintelligible document is transmitted without the operator at the source terminal knowing the fact that the transmission is failure and the transmitted image is unintelligible.
Thus, the conventional facsimile transmission system adapted for performing facsimile transmission via more than one transmission medium suffers from a problem that the facsimile transmission has to be repeated a number of times in order to complete transmission of even one page of the document. Otherwise, one has to accept poor quality of transmission in which a substantial part of the document may be dropped or unintelligible.